1. Field of the Invention
The present invention relates to a flash apparatus for photography or the like, and to a camera having the flash apparatus.
2. Description of Related Art
A conventional flash apparatus will be schematically explained as to its arrangement and operation with reference to FIGS. 4 and 5. FIG. 4 is a circuit diagram showing the arrangement of the conventional flash apparatus. FIG. 5 is a flowchart showing the operation of the conventional flash apparatus in a flash mode.
First, a control circuit (not shown) operates a charge inhibition timer for interrupting an operation for charging a main capacitor 21 (step S401). Next, the control circuit applies an H level signal through a terminal a to start oscillation and further applies an L level signal (pulse signal) to a terminal b (step S402).
The H level signal applied to the terminal a acts as the base current of a transistor 9 through a resistor 6, which makes the transistor 9 conductive. As a result, one input terminal of a NOR circuit 12, which has been pulled up by an auxiliary power source Vcc 23 through a resistor 7, becomes an L level. In contrast, since the terminal b is momentarily set to an L level, the other end of the NOR circuit 12 also becomes an L level. With this operation, the output of the NOR circuit 12 becomes an H level, and potential is applied to a resistor 15.
Since this potential is connected to the gate terminal of an FET 14, the FET 14 is conducted by receiving a gate driving voltage. The conduction of the FET 14 causes a current to flow from a battery 1 to the primary winding P of an oscillation transformer 13. Thus, an electromotive force is induced in the secondary winding S of the oscillation transformer 13 so that a current flows through a loop composed of a high voltage rectifying diode 17, a main capacitor 21, and a rectifying element 16.
Since the cathode potential of the rectifying element 16 is lower than the anode potential thereof by about 0.7 V, a current flows from the auxiliary power source Vcc 23 through resistors 10 and 11. With this operation, since potential connected to a midpoint between the resistors 10 and 11 becomes an L level, the L level can be maintained even after the terminal b momentarily becomes the L level.
When the conduction of the FET 14 is continued and the magnetic flux of the core of the oscillation transformer 13 is saturated, a counter electromotive force is generated and the current charged in the main capacitor 21 is exhausted as well as no current flows from the auxiliary power source Vcc 23 to the resistors 10 and 11, which sets one input terminal of the NOR circuit 12 to an H level so that the output from the NOR circuit 12 becomes an L level.
When the output from the NOR circuit 12 becomes the L level, the gate charge of the FET 14 becomes an L level, which makes the FET 14 non-conductive momentarily. While the counter electromotive force is generated by receiving a reverse bias due to the capacitance of the high voltage rectifying diode 17, potential higher than that of the auxiliary power source Vcc 23 is generated to the cathode of the rectifying element 16.
When the magnetic flux of the core is reduced and the counter electromotive force is reversed to a forward oscillation voltage, the rectifying element 16 receives a bias voltage again and the cathode potential thereof is reduced, whereby a current flows from the auxiliary power source 23 to the resistor 11 through the resistor 10 and the input terminal of the NOR circuit 12 becomes an L level, which conducts the FET 14 again.
Oscillation is executed by repeating the above actions so that the voltage charged in the main capacitor 21 is increased.
While the main capacitor 21 is charged, the control circuit causes a voltage detecting circuit 18 to output the information of the voltage charged in the main capacitor 21 through a terminal d and determines whether or not the charged voltage has reached a predetermined charge completion voltage (step S403).
When the voltage charged in the main capacitor 21 has reached the predetermined charge completion voltage, the control circuit interrupts the charging operation of the main capacitor 21 by stopping the H level signal outputted through the terminal a (step S405). Next, the control circuit completes the charging operation by setting a charge completion flag (step S406).
Otherwise, when the voltage charged in the main capacitor 21 has not reached the charge completion voltage, the control circuit determines whether or not the above-mentioned charge inhibition timer has reached a predetermined count completion value (step S404). When the charge inhibition timer has not reached the predetermined charge completion value, the control circuit returns to step S403, whereas when the charge inhibition timer has reached the predetermined charge completion value, the control circuit interrupts the charging operation of the main capacitor 21 by stopping the H level signal outputted through the terminal a (step S407). Next, the control circuit completes the charging operation by setting a charge NG flag indicating the charge NG (step S408).
However, as the voltage of the battery drops, a power source voltage compensating circuit temporarily stops the oscillating operation of an oscillation circuit temporarily. As a result, a secondary current is reduced. Further, when the power source voltage compensating circuit stops the oscillating operation just before the oscillation transformer is saturated, the secondary current is more reduced. As a result, there is a possibility that the oscillating operation of the voltage boosting circuit is perfectly stopped.
Further, the conventional flash apparatus measures the voltage charged in the main capacitor every time a predetermined time passes and detects a problem in an charging operation from a result of the measurement. However, the conventional flash apparatus cannot detect abnormal states, for example, discharge of a large current due to short-circuit of the main capacitor, breakage of a charged voltage detection wiring, a voltage excessively charged in the main capacitor, and the like at an early time.
An object according to a first aspect of the invention is to provide a flash apparatus capable of detecting the operating state of a voltage boosting circuit at an early time and controlling the operation of the voltage boosting circuit according to the operating state and to provide a camera having the flash apparatus.
Further, the conventional flash apparatus ordinarily emits a discharge tube to illuminate, for example, a subject in such a manner that the voltage of a battery is increased using a bipolar transistor as an oscillation transistor, a charge having an increased voltage is accumulated in a main capacitor and discharged through the discharge tube.
Since the bipolar transistor has a low operating voltage and a present DC/DC converter is of a current feedback type, it is possible to flow the current charged in the main capacitor through a loop between the base and emitter of the oscillation transistor. Thus, there is an advantage that the number of parts can be considerably reduced.
However, as the sizes of cameras become smaller, the number of batteries used thereby is reduced, and, at present, cameras employ 3 V power sources in many cases, while they conventionally employed 6 V power sources.
Further, recent compact cameras are required to have large guide numbers to be provided with a zoom function and to expand a photographing region.
Therefore, bipolar transistors used for oscillation are required to have such a performance that they have higher hEF, a lower saturated voltage VCE (sat) between an emitter and a collector and further a larger current-carrying capacity. Accordingly, at present, bipolar transistors, which can satisfy these requirements, are limited.
In contrast with these bipolar transistors, FETs acting as insulated gate type transistors recently have greatly improved performances with a gate driving voltage for conducting them reduced to 2.5 to 4 V. Further, there are available devices having an operating resistance of about 20 to 30 mxcexa9 in conduction. Further, since many devices have a current-carrying capacity of 5 to 10 A, they can be sufficiently used as oscillation devices for flash apparatuses.
FETs can sufficiently cope with a tendency, which is expected hereinafter, of further reducing a voltage of power sources such as batteries and the like because they can be operated as long as a gate driving voltage is guaranteed, while bipolar transistors are required to have the high hFET.
FIG. 26 shows a conventional example using an FET. Reference numeral 301 denotes a battery acting as a power source, reference numeral 360 denotes a power source stabilizing capacitor connected to both the ends of the battery 301, reference numeral 312 denotes a transformer for increasing the voltage of the battery 301 one terminal f of the primary winding which is connected to the positive electrode of the battery 301, the other terminal f of the primary winding is connected to the drain of an N-channel field effect transistor (hereinafter, abbreviated as FET) 313 acting as a switch element (which will be to be described later). Further, one terminal h of the secondary winding of the FET 313 is connected to the anode of a rectifying diode 315 to be described later and the other terminal i of the secondary winding is connected to the base of a PNP transistor 381 (which will be to be described later).
Further, the source of the FET 313 is connected to the negative electrode of the battery 301, and the gate thereof is connected to the output of a logic circuit 337 acting as one of active elements serving as a control unit (which will be described later).
The logic circuit 337 is composed of, for example, an AND logic. It is to be noted that the active element is an ordinary IC which uses an output Vcc from a constant voltage circuit (power source) 420 (which will be described later) as a power source. The active element is arranged to be a driver circuit for driving the gate of the above-mentioned FET 313, to stabilize the gate voltage and to improve the rise and fall characteristics of an on-off time control.
One of the inputs of the active element 337 receives the output of a current-voltage conversion unit 348 to be described later, and the other of the inputs is arranged as a signal input from a signal terminal CGCOM. The output of the active element 337 of the control unit is connected to the gate of the FET 313.
The active element 337 of the control unit outputs a high level (hereinafter, abbreviated as xe2x80x9cHLxe2x80x9d) signal only when both the outputs of the current-voltage conversion unit 348 and the signal CGCOM are at xe2x80x9cHLxe2x80x9d.
The active element 337 outputs a low level (hereinafter abbreviated to xe2x80x9cLLxe2x80x9d) signal when the signal CGCOM becomes xe2x80x9cLLxe2x80x9d. The current-voltage conversion unit 348 is composed of a PNP transistor 381, a protective resistor 382, a capacitor 383 and a resistor 384.
The current-voltage conversion unit 348 is arranged to convert a current flowing from the secondary winding of the transformer 312 to a main capacitor 320 into a driving voltage for the switch element. The emitter of the PNP transistor 381 is connected to the output Vcc of a constant voltage circuit 420 (which will be described later) and the base thereof is connected to the terminal i of the secondary winding of the transformer 312. The protective resistor 382 is connected between the emitter and base of the PNP transistor 382, and the capacitor 383 also is connected therebetween. Further, the resistor 384 has one end connected to the base of the PNP transistor 381 and the other end connected to the negative electrode of the battery 301. When a base current of the PNP transistor 381 is pulled during oscillation, a current which is proportional to the base current flows between the emitter and collector of the PNP transistor 381 to bring about an electromotive force at the resistor 384, so that the current is converted into a voltage. A resistor 361 is a flow limit resistor connected between the base of the PNP transistor 381 and the CGST terminal of a control circuit 425.
A resistor 347 has one end connected to the CGCOM terminal of the control circuit 425 and the other terminal connected to the input of the active element 337 of the control unit.
Reference numeral 341 denotes an operation stabilizing capacitor connected between the output Vcc of the constant voltage circuit 420 and the negative electrode of the battery 301, and reference numeral 393 denotes an output voltage maintaining unit as a known output voltage maintaining circuit for maintaining the output voltage. (Vcc voltage) of the constant voltage circuit 420 when the voltage of the battery drops in a charging operation. When an input voltage (from the battery) suddenly drops like in the case of charging, the output voltage maintaining unit 393 cannot maintain its output voltage. Therefore, the output voltage maintaining unit 393 is arranged to cut off the control signal CGCOM when the input voltage becomes lower than a power-source-voltage cut-off level (hereinafter, abbreviated as Vref voltage) set by the constant voltage circuit 420.
Reference numeral 302 denotes a resistor having one end connected to the positive electrode of the battery 301 and the other end connected to the non-inverting input terminal of a comparator 304 (which will be described later). Reference numeral 303 denotes a capacitor having one end connected to the non-inverting input terminal of the comparator 304 and the other end connected to the negative electrode of the battery 301. This capacitor 303 is arranged to have hysteresis with respect to its input. The comparator 304 is arranged, in this case, to have an open-collector-type output. To the non-inverting input terminal of the comparator 304 is connected the other end of the resistor 302 and one end of the capacitor 303. The inverting input terminal of the comparator 304 is supplied with the power-source-voltage cut-off level voltage (Vref voltage) from the constant voltage circuit 420. Further, the output of the comparator 304 is connected to one end of the resistor 347 and the input of the active element 337.
A specific operation of the conventional example is such that, first, an input voltage is determined as the power-source-voltage cut-off level voltage Vref which is set by the constant voltage circuit 420 and supplied to the inverting input terminal of the comparator 304.
The non-inverting input terminal of the comparator 304 detects the voltage of the battery 301 through the resistor 302. When an voltage increasing operation is started in this state, the control circuit 425 changes the oscillation start signal CGCOM from xe2x80x9cLLxe2x80x9d to xe2x80x9cHLxe2x80x9d. The input of the active element 337 becomes Thereafter, a one-shot signal, which changes from an xe2x80x9cOPENxe2x80x9d state to an xe2x80x9cLLxe2x80x9d state for a very short period of time, is outputted from the CGST terminal of the control circuit 425 through the resistor 361. With this operation, the base current of the PNP transistor 381 is pulled.
When the base of the PNP transistor 381 of the current-voltage conversion unit 348 is pulled, the transistor 381 is turned on, a current flows from its emitter connected to the Vcc voltage terminal to its collector, and a voltage is generated to both the ends of the resistor 384.
Thus, both the inputs of the active element 337 become xe2x80x9cHLxe2x80x9d, whereby the output thereof becomes xe2x80x9cHLxe2x80x9d. As a result, the FET 313 is turned on, and the current of the battery 301 flows from the drain to source of the FET 313 through the terminals e and b of the primary winding of the transformer 312, which causes the current to flow on the primary side of the transformer 312 to thereby generate a voltage, which is proportional to a winding ratio, to the secondary side thereof as well as pulls the base current of the PNP transistor 381 connected to the terminal h of the secondary winding of the transformer 312.
From the Vcc constant voltage source, a current is supplied to the transformer 312 through the emitter and base of the PNP transistor 381 and to the main capacitor 320 through the high voltage rectifying diode 315. As the current increases, the transformer 312 is magnetically saturated and the current is rapidly attenuated. With this operation, the base current of the PNP transistor 381 is not pulled and a current which is proportional thereto flows between the emitter and collector of the PNP transistor 381 to thereby drop the voltage by the resistor 384 (voltage-current conversion). Therefore, the output of the active element 337 becomes xe2x80x9cLLxe2x80x9d, and the gate of the FET 313 becomes xe2x80x9cLLxe2x80x9d so as to turn off it, and the supply of the electric power from the battery 301 to the terminal e of the primary wining of the transformer 312 is cut off.
However, since the secondary current of the transformer 312 executes damped oscillation, the voltage at both the ends of the resistor 384 in the current-voltage conversion unit 348 is increased again and the inputs of the active element 337 of the control unit become xe2x80x9cHLxe2x80x9d together, the output of the active element 337 of the control unit becomes xe2x80x9cHLxe2x80x9d. As a result, the FET 313 is turned on and the current of the battery power source 301 flows to the drain and source of the FET 313 through the terminals f and g of the transformer 312 so as to flow a current to the primary side thereof, so that oscillation is repeated similarly to the above-mentioned. Thus, a charge is accumulated in the main capacitor 320 and a voltage is increased.
At this time, the voltage of the battery 301 rapidly drops because the FET 313 is turned on. However, when the voltage drops below the Vref voltage of the input of the comparator 304, the comparator 304 is reversed from an open state to an xe2x80x9cLLxe2x80x9d state, thereby reducing the output of the charge control signal CGCOM of the control circuit 425. Thus, one input of the active element 337 becomes xe2x80x9cLLxe2x80x9d to set the output thereof to xe2x80x9cLLxe2x80x9d, so that the FET 313 is turned off, and oscillation begins to be interrupted.
With the oscillation stopped, the voltage of the battery 301 ceases to drop and comes to recover. With the voltage of the battery 301 thus coming to recover, when the voltage of the battery 301 exceeds the voltage Vref of the input of the comparator 304, the output of the comparator 304 changes from the xe2x80x9cLLxe2x80x9d state to the open state. Accordingly, the input of the active element 337 becomes xe2x80x9cHLxe2x80x9d to bring the output thereof to xe2x80x9cHLxe2x80x9d again. Therefore, the FET 313 is turned on to resume oscillation. The voltage of the power source is thus prevented from becoming less than a predetermined voltage by repeating the actions in above manner.
However, in the above-mentioned conventional flash apparatus, the charged current, which flows to the main capacitor 320, flows to a loop composed of the terminal h of the secondary winding of the transformer 312, the main capacitor 320, the constant voltage circuit 420, the emitter to base of the transistor 381, and the terminal i of the secondary winding of the transformer 312.
Since this current flows through the constant voltage circuit 420, a large current capacity is required to the constant voltage circuit 420. When a power source voltage is 6 V, an ordinary flash apparatus is supplied with an average current of about 6 A from a battery. When there is no loss, a current of 1/(secondary winding ratio of a transformer) flows on the secondary side thereof.
Ib=Ic/n
where, Ib shows a secondary current, Ic shows a primary current, and n shows a winding ratio.
When the winding ratio of the transformer is 100 (n), it can be calculated that the transformer has a supply capacity of about 60 mA. There is a drawback that a larger current supply capacity is required when currents consumed by other control circuits are added.
Accordingly, an object according to a second aspect of the invention is to provide a flash apparatus capable of suppressing a current supply capacity.
One aspect of the invention resides in that when it is determined that the oscillation of a voltage boosting circuit is stopped before it is controlled so as to stop, it is possible to continuously charge a voltage to a main capacitor by causing the voltage boosting circuit to continue oscillation by providing a control circuit for oscillating the voltage boosting circuit again.
Further, another aspect of the invention is a flash apparatus which includes a charged current detecting circuit for detecting the current charged in a main capacitor using an auxiliary power source as a power source. The charged current detecting circuit is simply arranged with a passive element for bypassing a part of the current charged in the main capacitor from a loop through the above-mentioned auxiliary power source so as to suppress electric power supplied from the auxiliary power source to a low level.
These and further aspects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.